This invention relates to a gene which predisposes individuals to breast and ovarian cancer. More specifically, this invention relates to a specific mutation in the BRCA2 gene. In addition, it also relates to a method for detecting the presence of the mutation.
BRCA2, located on chromosome 13q 12-q13, consists of over 70 kb of genomic DNA. The coding sequence produces a protein of 3,418 amino acids. Although most of the exons are small, exons 10 and 11 represent approximately 60% of the entire coding region. Germline mutations of BRCA2 are predicted to account for approximately 35% of families with multiple case, early onset female breast cancer, and they are also associated with an increased risk of male breast cancer, ovarian cancer, prostate cancer and pancreatic cancer.
The location of one or more mutations in the BRCA2 gene provides a promising approach to reducing the high incidence and mortality associated with breast and ovarian cancer through the early detection of women at high risk. These women, once identified, can be targeted for more aggressive prevention programs. Screening is carried out by a variety of methods which include karyotyping, probe binding and DNA sequencing. In such cases where one or only a few known mutations are responsible for the disease, methods for detecting the mutations are targeted to the site within the gene at which they are known to occur.
There is a need in the art to identify mutations in the BRCA2 gene. Identification of mutations of the BRCA2 gene and protein would allow more widespread diagnostic screening for hereditary breast and ovarian cancer than is currently possible.
The present invention is based on the discovery of a two base pair deletion of nucleotide 6495 of the published BRCA2 cDNA sequence which is associated with susceptibility to and development of breast and ovarian cancer.
It is an object of the invention to provide a method for determining a predisposition or higher susceptibility to breast and ovarian cancer.
It is another object of the invention to provide primers for detecting and amplifying a region of DNA which contains the 6495delGC mutation.
The present invention is based on the discovery of a two base pair deletion of nucleotide 6495 of the published BRCA2 cDNA sequence. This deletion mutation is referred to as 6495delGC. The BRCA2 gene is a tumor suppressor gene associated with breast and ovarian cancer.
The 6495delGC interrupts the normal reading frame of the BRCA2 transcript, resulting in the appearance of an in-frame terminator TAG at codon position 2090. This mutation is, therefore, predicted to result in a truncated, and most likely, non-functional protein.
Useful DNA molecules according to the invention are those which will specifically hybridize to BRCA2 sequences in the region of the 6495delGC mutation. Typically these are 17 to 20 nucleotides in length and have the nucleotide sequence corresponding to the region of the 6495delGC mutation at nucleotides 6495 of the BRCA2 cDNA sequence. Such molecules can be labeled, according to any technique known in the art, such as with radiolabels, fluorescent labels, enzymatic labels, sequence tags, etc. According to another aspect of the invention, the DNA molecules contain the 6495delGC mutation. Such molecules can be used as allele-specific oligonucleotide probes to track a particular mutation through a family.
Blood samples can be tested to determine whether the BRCA2 gene contains the 6495delGC mutation. In one embodiment of the invention a pair of isolated oligonucleotide primers are provided.
BRCA2-11F: 5xe2x80x2-TAC AGC AAG TGG AAA GC-3xe2x80x2(SEQ ID NO: 1), and BRCA2-11-R: 5xe2x80x2-AAG TTT CAG TTT TAC CAA T-3xe2x80x2(SEQ ID NO:2). The designation BRCA2-11 refers to a sequence in exon 11 of the BRCA2 gene. F and R refer to forward and reverse. The oligonucleotide primers are useful in direct amplification of a target polynucleotide prior to sequencing. These unique BRCA2 exon 11 oligonucleotide primers were designed and produced at Oncormed based upon identification of the 6495delGC mutation.
In another embodiment of the invention a pair of isolated allele specific oligonucleotides are provided.
5xe2x80x2-GAA CTG AGC ATA GTC TT-3xe2x80x2(SEQ ID NO:3), and
5xe2x80x2-GAA CTG AAT AGT CTT CA-3xe2x80x2(SEQ ID NO:4).
The allele specific oligonucleotides are useful in diagnosis of a subject at risk of having breast or ovarian cancer. The allele specific oligonucleotides hybridize with a target polynucleotide sequence containing the 6495delGC mutation. 5xe2x80x2-GAA CTG AGC ATA GTC TT-3xe2x80x2(SEQ ID NO:3) hybridizes preferentially to the wildtype sequence and is useful as a control sequence. 5xe2x80x2-GAA CTG AAT AGT CTT CA-3xe2x80x2(SEQ ID NO:4) is designed to hybridize preferentially to the mutant sequence.
The term xe2x80x9csubstantially complementary toxe2x80x9d or xe2x80x9csubstantially the sequencexe2x80x9d refers to (e.g., SEQ ID NO:3 and SEQ ID NO:4) sequences which hybridize to the sequences provided under stringent conditions and/or sequences having sufficient homology with SEQ ID NO:3 and SEQ ID NO:4, such that the allele specific oligonucleotides of the invention hybridize to the sequence. The term xe2x80x9cisolatedxe2x80x9d as used herein includes oligonucleotides substantially free of other nucleic acids, proteins, lipids, carbohydrates or other materials with which they may be associated. Such association being either in cellular material or in a synthesis medium. A xe2x80x9ctarget polynucleotidexe2x80x9d refers to the nucleic acid sequence of interest e.g., the BRCA2 encoding polynucleotide. Other primers which can be used for primer hybridization will be known or readily ascertainable to those of skill in the art.
The primers of the invention embrace oligonucleotides of sufficient length and appropriate sequence so as to provide initiation of polymerization on a significant number of nucleic acids in the polymorphic locus. Specifically, the term xe2x80x9cprimerxe2x80x9d as used herein refers to a sequence comprising two or more deoxyribonucleotides or ribonucleotides, preferably more than three, and more preferably more than eight and most preferably at least 20 nucleotides of the BRCA2 gene wherein said DNA sequence contains the 6495delGC mutation relative to BRCA2 contained in SEQ ID NO""s:3 and 4. Environmental conditions conducive to synthesis include the presence of nucleoside triphosphates and an agent for polymerization, such as DNA polymerase, and a suitable temperature and pH. The primer is preferably single stranded for maximum efficiency in amplification, but may be double stranded. If double stranded, the primer is first treated to separate its strands before being used to prepare extension products. The primer must be sufficiently long to prime the synthesis of extension products in the presence of the inducing agent for polymerization. The exact length of the primer will depend on many factors, including temperature, buffer, and nucleotide composition. The oligonucleotide primer typically contains 12-20 or more nucleotides, although it may contain fewer nucleotides.
Primers of the invention are designed to be xe2x80x9csubstantiallyxe2x80x9d complementary to each strand of the genomic locus to be amplified. This means that the primers must be sufficiently complementary to hybridize with their respective strands under conditions which allow the agent for polymerization to perform. In other words, the primers should have sufficient complementarity with the 5xe2x80x2 and 3xe2x80x2 sequences flanking the mutation to hybridize therewith and permit amplification of the genomic locus.
Oligonucleotide primers of the invention are employed in the amplification process which is an enzymatic chain reaction that produces exponential quantities of polymorphic locus relative to the number of reaction steps involved. Typically, one primer is complementary to the negative (xe2x88x92) strand of the polymorphic locus and the other is complementary to the positive (+) strand. Annealing the primers to denatured nucleic acid followed by extension with an enzyme, such as the large fragment of DNA polymerase I (Klenow) and nucleotides, results in newly synthesized + and xe2x88x92 strands containing the target polymorphic locus sequence. Because these newly synthesized sequences are also templates, repeated cycles of denaturing, primer annealing, and extension results in exponential production of the region (i.e., the target polymorphic locus sequence) defined by the primers. The product of the chain reaction is a discreet nucleic acid duplex with termini corresponding to the ends of the specific primers employed.
The oligonucleotide primers of the invention may be prepared using any suitable method, such as conventional phosphotriester and phosphodiester methods or automated embodiments thereof. In one such automated embodiment, diethylphosphoramidites are used as starting materials and may be synthesized as described by Beaucage, et. al., Tetrahedron Letters, 22:1859-1862 (1981). One method for synthesizing oligonucleotides on a modified solid support is described in U.S. Pat. No. 4,458,066.
Any nucleic acid specimen, in purified or nonpurified form, can be utilized as the starting nucleic acid or acids, providing it contains, or is suspected of containing, the specific nucleic acid sequence containing the polymorphic locus. Thus, the process may amplify, for example, DNA or RNA, including messenger RNA, wherein DNA or RNA may be single stranded or double stranded. In the event that RNA is to be used as a template, enzymes, and/or conditions optimal for reverse transcribing the template to DNA would be utilized. In addition, a DNA-RNA hybrid which contains one strand of each may be utilized. A mixture of nucleic acids may also be employed, or the nucleic acids produced in a previous amplification reaction herein, using the same or different primers may be so utilized. The specific nucleic acid sequence to be amplified, i.e., the polymorphic locus, may be a fraction of a larger molecule or can be present initially as a discrete molecule, so that the specific sequence constitutes the entire nucleic acid. It is not necessary that the sequence to be amplified be present initially in a pure form; it may be a minor fraction of a complex mixture, such as contained in whole human DNA.
DNA utilized herein may be extracted from a body sample, such as blood, tissue material and the like by a variety of techniques such as that described by Maniatis, et. al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor, N.Y., pp. 280-281 (1982). If the extracted sample is impure, it may be treated before amplification with an amount of a reagent effective to open the cells, or animal cell membranes of the sample, and to expose and/or separate the strand(s) of the nucleic acid(s). This lysing and nucleic acid denaturing step to expose and separate the strands will allow amplification to occur much more readily.
The deoxyribonucleotide triphosphates dATP, dCTP, dGTP, and dTTP are added to the synthesis mixture, either separately or together with the primers, in adequate amounts and the resulting solution is heated to about 90xc2x0-100xc2x0 C. from about 1 to 10 minutes, preferably from 1 to 4 minutes. After this heating period, the solution is allowed to cool, which is preferable for the primer hybridization. To the cooled mixture is added an appropriate agent for effecting the primer extension reaction (called herein xe2x80x9cagent for polymerizationxe2x80x9d), and the reaction is allowed to occur under conditions known in the art. The agent for polymerization may also be added together with the other reagents if it is heat stable. This synthesis (or amplification) reaction may occur at room temperature up to a temperature above which the agent for polymerization no longer functions. Thus, for example, if DNA polymerase is used as the agent, the temperature is generally no greater than about 40xc2x0 C. Most conveniently the reaction occurs at room temperature.
The agent for polymerization may be any compound or system which will function to accomplish the synthesis of primer extension products, including enzymes. Suitable enzymes for this purpose include, for example, E. coli DNA polymerase I, Klenow fragment of E. coli DNA polymerase, polymerase muteins, reverse transcriptase, other enzymes, including heat-stable enzymes (i.e., those enzymes which perform primer extension after being subjected to temperatures sufficiently elevated to cause denaturation), such as Taq polymerase. Suitable enzyme will facilitate combination of the nucleotides in the proper manner to form the primer extension products which are complementary to each polymorphic locus nucleic acid strand. Generally, the synthesis will be initiated at the 3xe2x80x2 end of each primer and proceed in the 5xe2x80x2 direction along the template strand, until synthesis terminates, producing molecules of different lengths.
The newly synthesized strand and its complementary nucleic acid strand will form a double-stranded molecule under hybridizing conditions described above and this hybrid is used in subsequent steps of the process. In the next step, the newly synthesized double-stranded molecule is subjected to denaturing conditions using any of the procedures described above to provide single-stranded molecules.
The steps of denaturing, annealing, and extension product synthesis can be repeated as often as needed to amplify the target polymorphic locus nucleic acid sequence to the extent necessary for detection. The amount of the specific nucleic acid sequence produced will accumulate in an exponential fashion. PCR. A Practical Approach, ILR Press, Eds. M. J. McPherson, P. Quirke, and G. R. Taylor (1992).
The amplification products may be detected by analyzing it by Southern blots without using radioactive probes. In such a process, for example, a small sample of DNA containing a very low level of the nucleic acid sequence of the polymorphic locus is amplified, and analyzed via a Southern blotting technique or similarly, using dot blot analysis. The use of non-radioactive probes or labels is facilitated by the high level of the amplified signal. Alternatively, probes used to detect the amplified products can be directly or indirectly detectably labeled, for example, with a radioisotope, a fluorescent compound, a bioluminescent compound, a chemiluminescent compound, a metal chelator or an enzyme. Those of ordinary skill in the art will know of other suitable labels for binding to the probe, or will be able to ascertain such, using routine experimentation.
Sequences amplified by the methods of the invention can be further evaluated, detected, cloned, sequenced, and the like, either in solution or after binding to a solid support, by any method usually applied to the detection of a specific DNA sequence such as PCR, oligomer restriction (Saiki, et. al., Bio/Technology, 3:1008-1012, 1985), allele-specific oligonucleotide (ASO) probe analysis (Conner, et. al., Proc. Nat. Acad. Sci. U.S.A., 80:278, 1983), oligonucleotide ligation assays (OLAS) (Landgren, et. al., Science, 241:1007, 1988), and the like. Molecular techniques for DNA analysis have been reviewed (Landgren, et. al., Science, 242:229-237, 1988).
Preferably, the method of amplifying is by PCR, as described herein and as is commonly used by those of ordinary skill in the art. Alternative methods of amplification have been described and can also be employed as long as the BRCA2 locus amplified by PCR using primers of the invention is similarly amplified by the alternative means. Such alternative amplification systems include but are not limited to self-sustained sequence replication, which begins with a short sequence of RNA of interest and a T7 promoter. Reverse transcriptase copies the RNA into cDNA and degrades the RNA, followed by reverse transcriptase polymerizing a second strand of DNA. Another nucleic acid amplification technique is nucleic acid sequence-based amplification (NASBA) which uses reverse transcription and T7 RNA polymerase and incorporates two primers to target its cycling scheme. NASBA can begin with either DNA or RNA and finish with either, and amplifies to 108 copies within 60 to 90 minutes. Alternatively, nucleic acid can be amplified by ligation activated transcription (LAT). LAT works from a single-stranded template with a single primer that is partially single-stranded and partially double-stranded. Amplification is initiated by ligating a cDNA to the promoter olignucleotide and within a few hours, amplification is 108 to 109 fold. The QB replicase system can be utilized by attaching an RNA sequence called MDV-1 to RNA complementary to a DNA sequence of interest. Upon mixing with a sample, the hybrid RNA finds its complement among the specimen""s mRNAs and binds, activating the replicase to copy the tagalong sequence of interest. Another nucleic acid amplification technique, ligase chain reaction (LCR), works by using two differently labeled halves of a sequence of interest which are covalently bonded by ligase in the presence of the contiguous sequence in a sample, forming a new target. The repair chain reaction (RCR) nucleic acid amplification technique uses two complementary and target-specific oligonucleotide probe pairs, thermostable polymerase and ligase, and DNA nucleotides to geometrically amplify targeted sequences. A 2-base gap separates the oligonucleotide probe pairs, and the RCR fills and joins the gap, mimicking normal DNA repair. Nucleic acid amplification by strand displacement activation (SDA) utilizes a short primer containing a recognition site for hincII with short overhang on the 5xe2x80x2 end which binds to target DNA. A DNA polymerase fills in the part of the primer opposite the overhang with sulfur-containing adenine analogs. HincII is added but only cuts the unmodified DNA strand. A DNA polymerase that lacks 5xe2x80x2 exonuclease activity enters at the cite of the nick and begins to polymerize, displacing the initial primer strand downstream and building a new one which serves as more primer. SDA produces greater than 107-fold amplification in 2 hours at 37xc2x0 C. Unlike PCR and LCR, SDA does not require instrumented Temperature cycling. Another amplification system useful in the method of the invention is the QB Replicase System. Although PCR is the preferred method of amplification of the invention, these other methods can also be used to amplify the BRCA2 locus as described in the method of the invention.
In another embodiment of the invention a method is provided for diagnosing a subject having a predisposition or higher susceptibility to (at risk of) breast or ovarian cancer comprising sequencing a target nucleic acid of a sample from a subject by dideoxy sequencing following amplification of the target nucleic acid.
In another embodiment of the invention a method is provided for diagnosing a subject having a predisposition or higher susceptibility to (at risk of) breast or ovarian cancer, comprising contacting a target nucleic acid of a sample from a subject with a reagent that detects the presence of the 6495delGC mutation and detecting the mutation.
In another embodiment of the invention a method is provided for characterizing a tumor. One method comprises sequencing the target nucleic acid isolated from the tumor to determine if the 6495delGC has occured. Sanger, F., et. al., J Mol. Biol. 142:1617 (1980).
Another method comprises contacting a target nucleic acid of a sample from a subject with a reagent that detects the presence of the 6495delGC mutation and detecting the mutation. A number of hybridization methods are well known to those skilled in the art. Many of them are useful in carrying out the invention.
The materials for use in the method of the invention are ideally suited for the preparation of a diagnostic kit. Such a kit may comprise a carrier means being compartmentalized to receive in close confinement one or more container means such as vials, tubes, and the like, each of the container means comprising one of the separate elements to be used in the method. For example, one of the container means may comprise means for amplifying BRCA2 DNA, said means comprising the necessary enzyme(s) and oligonucleotide primers for amplifying said target DNA from the subject. The oligonucleotide primers include primers having a sequence:
BRCA-2-11F: 5xe2x80x2-TAC AGC AAG TGG AAA GC-3xe2x80x2(SEQ ID NO:1) or
5xe2x80x2-AAG TTT CAG TTT TAC CAA T-3xe2x80x2(SEQ ID NO:2)
or primer sequences substantially complementary or substantially homologous thereto. The target flanking 5xe2x80x2 and 3xe2x80x2 polynucleotide sequence has substantially the sequence selected from the group consisting of:
5xe2x80x2-GAA CTG AGC ATA GTC TT-3xe2x80x2(SEQ ID NO: 3), and
5xe2x80x2-GAA CTG AAT AGT CTT CA-3xe2x80x2(SEQ ID NO:4)
and sequences substantially complementary or homologous thereto. Other oligonucleotide primers for amplifying BRCA2 will be known or readily ascertainable to those of skill in the art.
The following definitions are provided for the purpose of understanding this invention.
xe2x80x9cCoding sequencexe2x80x9d or xe2x80x9cDNA coding sequencexe2x80x9d refers to those portions of a gene which, taken together, code for a peptide (protein), or which nucleic acid itself has function.
xe2x80x9cPrimerxe2x80x9d as used herein refers to a sequence comprising about 20 or more nucleotides of a gene used to initiate DNA synthesis via the PCR.

A xe2x80x9ctarget polynucleotidexe2x80x9d refers to the nucleic acid sequence of interest e.g., the BRCA2 encoding polynucleotide.
xe2x80x9cConsensusxe2x80x9d means the most commonly occurring in the population.
xe2x80x9cSubstantially complementary toxe2x80x9d refers to probe or primer sequences which hybridize to the sequences provided under stringent conditions and/or sequences having sufficient homology with test polynucleotide sequences, such that the allele specific oligonucleotide probe or primers hybridize to the test polynucleotide sequences to which they are complimentary.
xe2x80x9cIsolatedxe2x80x9d as used herein refers to substantially free of other nucleic acids, proteins, lipids, carbohydrates or other materials with which they may be associated. Such association is typically either in cellular material or in a synthesis medium.
xe2x80x9cSequence variationxe2x80x9d as used herein refers to any difference in nucleotide sequence between two different oligonucleotide or polynucleotide sequences.
xe2x80x9cPolymorphismxe2x80x9d as used herein refers to a sequence variation in a gene which is not associated with pathology.
xe2x80x9cMutationxe2x80x9d as used herein refers to an altered genetic sequence which results in the gene coding for a non-functioning protein or a protein with substantially reduced or altered function. Generally, a deleterious mutation is associated with pathology or the potential for pathology. The mutations identified herein result in a premature stop codon.
xe2x80x9cPre-determined sequence variationxe2x80x9d as used herein refers to a nucleotide sequence that is designed to be different than the corresponding sequence in a reference nucleotide sequence. A predetermined sequence variation can be a known mutation in a gene.
xe2x80x9cAllele specific detection assayxe2x80x9d as used herein refers to an assay to detect the presence or absence of a pre-determined sequence variation in a test polynucleotide or oligonucleotide by annealing the test polynucleotide or oligonucleotide with a polynucleotide or oligonucleotide of pre-determined sequence such that differential DNA sequence based techniques or DNA amplification methods discriminate between normal and mutant.
xe2x80x9cSequence variation locating assayxe2x80x9d as used herein refers to an assay that detects a sequence variation in a test polynucleotide or oligonucleotide and localizes the position of the sequence variation to a sub-region of the test polynucleotide, without necessarily determining the precise base change or position of the sequence variation.
xe2x80x9cTargeted confirmatory sequencingxe2x80x9d as used herein refers to sequencing a polynucleotide in the region wherein a sequence variation has been located by a sequence variation locating assay in order to determine the precise base change and/or position of the sequence variation.
The invention in several of its embodiments includes:
Detection of Pre-determined Sequence Variations
Stage I analysis is used to determine the presence or absence of a pre-determined nucleotide sequence variation; preferably a known mutation or set of known mutations in the test gene. In accordance with the invention, such pre-determined sequence variations are detected by allele specific hybridization, a sequence-dependent-based technique which permits discrimination between normal and mutant alleles. An allele specific assay is dependent on the differential ability of mismatched nucleotide sequences (e.g., normal:mutant) to hybridize with each other, as compared with matching (e.g., normal:normal or mutant:mutant) sequences.
Detection of Pre-determined Sequence Variations Using Allele Specific Hybridization
A variety of methods well-known in the art can be used for detection of pre-determined sequence variations by allele specific hybridization. Preferably, the test gene is probed with allele specific oligonucleotides (ASOs); and each ASO contains the sequence of a known mutation. ASO analysis detects specific sequence variations in a target polynucleotide fragment by testing the ability of a specific oligonucleotide probe to hybridize to the target polynucleotide fragment. Preferably, the oligonucleotide contains the mutant sequence (or its complement). The presence of a sequence variation in the target sequence is indicated by hybridization between the oligonucleotide probe and the target fragment under conditions in which an oligonucleotide probe containing a normal sequence does not hybridize to the target fragment. A lack of hybridization between the sequence variant (e.g., mutant) oligonucleotide probe and the target polynucleotide fragment indicates the absence of the specific sequence variation (e.g., mutation) in the target fragment. In a preferred embodiment the test samples are probed in a standard dot blot format. Each region within the test gene that contains the sequence corresponding to the ASO is individually applied to a solid surface, for example, as an individual dot on a membrane. Each individual region can be produced, for example, as a separate PCR amplification product using methods well-known in the art (see, for example, the experimental embodiment set forth in Mullis, K. B., 1987, U.S. Pat. No. 4,683,202). The use of such a dot blot format is described in detail in one of the examples below, detailing the Stage I analysis of the human BRCA2 gene to detect the presence or absence of eight different known mutations using eight corresponding ASOS.
Membrane-based formats that can be used as alternatives to the dot blot format for performing ASO analysis include, but are not limited to, reverse dot blot, (multiplex amplification assay), and multiplex allele-specific diagnostic assay (MASDA).
In a reverse dot blot format, oligonucleotide or polynucleotide probes having known sequences are immobilized on the solid surface, and are subsequently hybridized with the labeled test polynucleotide sample.
In a multiplex format, individual samples contain multiple target sequences within the test gene, instead of just a single target sequence. For example, multiple PCR products each containing at least one of the ASO target sequences are applied within the same sample dot. Multiple PCR products can be produced simultaneously in a single amplification reaction using the methods of Caskey et. al., U.S. Pat. No. 5,582,989. The same blot, therefore, can be probed by each ASO whose corresponding sequence is represented in the sample dots.
A MASDA format expands the level of complexity of the multiplex format by using multiple ASOs to probe each blot (containing dots with multiple target sequences). This procedure is described in detail in U.S. Pat. No. 5,589,330 by A. P. Shuber, and in Michalowsky et. al., American Journal of Human Genetics 59(4):A272, poster 1573 (October 1996), each of which is incorporated herein by reference in its entirety. First, hybridization between the multiple ASO probe and immobilized sample is detected. This method relies on the prediction that the presence of a mutation among the multiple target sequences in a given dot is sufficiently rare that any positive hybridization signal results from a single ASO within the probe mixture hybridizing with the corresponding mutant target. The hybridizing ASO is then identified by isolating it from the site of hybridization and determining its nucleotide sequence.
Suitable materials that can be used in the dot blot, reverse dot blot, multiplex, and MASDA formats are well-known in the art and include, but are not limited to nylon and nitrocellulose membranes.
When the target sequences are produced by PCR amplification, the starting material can be chromosomal DNA in which case the DNA is directly amplified. Alternatively, the starting material can be mRNA, in which case. the mRNA is first reversed transcribed into cDNA and then amplified according to the well known technique of RT-PCR (see, for example, U.S. Pat. No. 5,561,058 by Gelfand et. al.).
The methods described above are suitable for moderate screening of a limited number of sequence variations. However, with the need in molecular diagnosis for rapid, cost effective large scale screening, technologies have developed that integrate the basic concept of ASO, but far exceed the capacity for mutation detection and sample number. These alternative methods to the ones described above include, but are not limited to, large scale chip array sequence-based techniques. The use of large scale arrays allows for the rapid analysis of many sequence variants. A review of the differences in the application and development of chip arrays is covered by Southern, E. M., Trends In Genetics, 12:110-115 (March 1996) and Cheng et. al., Molecular Diagnosis, 1:183-200 (September 1996). Several approaches exist involving the manufacture of chip arrays. Differences include, but not restricted to: type of solid support to attach the immobilized oligonucleotides, labeling techniques for identification of variants and changes in the sequence-based techniques of the target polynucleotide to the probe.
A promising methodology for large-scale analysis on xe2x80x98DNA chipsxe2x80x99 is described in detail in Hacia et. al., Nature Genetics 14:441-447 (1996), which is hereby incorporated by reference in its entirety. As described in Hacia et. al., high density arrays of over 96,000 oligonucleotides, each 20 nucleotides in length, are immobilized to a single glass or silicon chip using light directed chemical synthesis. Contingent on the number and design of the oligonucleotide probe, potentially every base in a sequence can be interrogated for alterations. Oligonucleotides applied to the chip, therefore, can contain sequence variations that are not yet known to occur in the population, or they can be limited to mutations that are known to occur in the population.
Prior to hybridization with oligo probes on the chip, the test sample is isolated, amplified and labeled (e.g. fluorescent markers) by means well known to those skilled in the art. The test polynucleotide sample is then hybridized to the immobilized oligonucleotides. The intensity of sequence-based techniques of the target polynucleotide to the immobilized probe is quantitated and compared to a reference sequence. The resulting genetic information can be used in molecular diagnosis.
A common, but not limiting, utility of the xe2x80x98DNA chipxe2x80x99 in molecular diagnosis is screening for known mutations. However, this may impose a limitation to the technique by only looking at mutations that have been described in the field. The present invention allows allele specific hybridization analysis be performed with a far greater number of mutations than previously available. Thus, the efficiency and comprehensiveness of large scale ASO analysis will be broadened, reducing the need for cumbersome end-to-end sequence analysis, not only with known mutations but in a comprehensive manner all mutations which might occur as predicted by the principles accepted, and the cost and time associated with these cumbersome tests will be decreased.